928 



SCIENCE 



[N. S. Vol. XXX. No. 782 



The data presented hardly support the con- 

 tention of the author, however, as will be 

 shown below. He considers the parasite to 

 belong particularly with this species of aphis, 

 saying (p. 163) : " It appears that Lysiphlehus 

 does not perpetuate itself and maintain a 

 general distribution on these other hosts," 

 though it was first described from Missouri 

 in 1888, and this particular plant louse was 

 not recorded as far west as Indiana until 1890. 



He recognizes the importance of weather 

 conditions and states (p. 9) : " The green bug 

 continues to be active and reproductive at a 

 lower temperature than does this parasite," 

 thus giving the plant louse a start in the 

 spring, and his assistant, Professor Glenn, 

 says in the appendix on the " influence of 

 climate," referring to the plant louse (p. 180), 

 " it can not endure the high temperatures 

 which prevail in summer," and in reference to 

 the parasite (p. 182) : " During the hot months 

 of July and August, they decrease in numbers 

 because of a lack of hosts, since the green 

 bugs can not endure temperatures much above 

 100° F." 



Since the aphid appears thus to have a lower 

 optimum temperature it is most favorable to 

 the parasite to compare the shortest period of 

 development of each, which is for the aphid 

 5 days, for the parasite 7 days. Earlier in 

 the year the intervals given are 11.5 and 17.69 

 days, respectively. The large amount of data 

 make the average rate of reproduction appear 

 to be very reliable and for the purpose of cal- 

 culation we may use the round numbers 2 

 per day for twenty-five days for the aphis and 

 8 per day for five days for the parasite. This 

 is favorable to the latter, the actual figures 

 being a total of 59 and 38 descendants, re- 

 spectively, for the summer condition. Three 

 fourths of the parasites are counted female, 

 though the actual number is only two thirds. 

 The following table gives the results of a 

 calculation made by me and based on the 

 figures given above. 



In explanation of the table it may be said 

 that only the large aphids are parasitized 

 and nearly 85 per cent, are young insects when 

 they are increasing at full capacity. Since one 



aphid produces 65,000 in 29 days and 95,000 in 



30 days, the difference between these figures 

 represents the descendants of the first two 

 young insects and the ratio between these 

 figures (68.5 per cent.) gives the reproductive 

 power of the adult insect, which alone is liable 

 to parasitization. The last column is ob- 

 tained by dividing the figures showing egg- 

 laying power by the number of adults (found 

 by counting back five lines in the first column) 

 and multiplying the quotient by the reproduc- 

 tive power of the adults. Thus the killing of 

 8 insects the second day affects the total of 

 progeny the same as the killing of 5.92 on the 

 first day, before the young had been produced. 

 Adding the last column gives us the total re- 

 sult of the work of the parasite for any date 

 within the month. The total for the month is 

 117.82. Had this number been killed the first 

 day, the result on the total progeny would 

 have been the same as that secured by killing 

 over 700,000 plant lice and attaching the mum- 

 mies of those dead plant lice to wheat stalks. 



